1. Field of the Invention
The present invention relates to methods and apparatus for recording and playing back ECG signals and more particularly to such methods and apparatus in which the ECG signals are processed to remove phase and magnitude distortion occurring during the recording and playback process.
2. Description of the Related Art
Prior art systems, known as Holter recorders, record ECG signals which are provided from a patient via electrodes to a small recorder carried by the patient. The ECG signals are continuously recorded on an analog tape typically over a 24 hour period. Several prior art systems use a conventional C60 analog cassette tape of the type typically used to record audio information. In order to do so, the tape must be slowed down approximately 50 times from normal audio speeds. Usually three channels of ECG data are recorded plus a fourth channel which includes a timing signal that is generated by the recorder.
After recording, the tape is played back on a device which runs the tape at high speed and processes the analog signals derived from the tape. Such processing typically includes analog filters which are intended to compensate for dropoff at both the low and high frequencies and for phase distortion which results from the magnetic recording and playback process. The filtered signals are digitized and further processed into a report for analysis by a physician.
As used herein, the term magnitude response refers to the manner in which a system for recording and playing back an ECG signal affects the magnitude of a signal as the signal frequency varies. The term phase response refers to the manner in which such a system affects the phase of a signal as the signal frequency varies. Magnitude response and phase response are referred to herein collectively as frequency response. An ideal system for recording and playing back ECG signals would have a substantially flat magnitude response and a linear phase response. Analog tape recording technology of the type described above suffers from several limitations which adversely affect both the magnitude and the phase response of prior art systems for recording and playing back ECG signals. Because the playback head on a device for scanning (i.e., playing back) recorded ECG signals detects only the rate of change of tape flux, there is no magnitude response at DC. The playback head in essence differentiates signals on the tape. This differentiation has the effect of phase shifting signals positive 90 degrees and attenuating low frequency signals. To recover the original information, the playback unit must integrate the output signal from the playback head. This integration process compensates for the differentiation by providing negative 90 degrees phase shift and amplification of low frequencies. In prior art systems, the integration process results in phase distortion and excessive amplification of low frequency noise (baseline wander).
High frequency magnitude response of the playback signal tends to drop off due to the physical limits of the playback head, recording head inductance, tape coating effects and also as a result of the slow rate of tape transport during recording. This effect may be compensated for by boosting the played back signal in the high frequencies. This has the effect of increasing both signal and noise in this range. Recent research indicates that there is clinically important information which can be obtained from an ECG signal above 40 Hz; increased noise in this range is therefore undesirable.
As mentioned above, prior art systems filter the signal derived from a tape playback head during scanning of the recorded tape in an effort to compensate for phase and magnitude distortion. Such systems suffer from several disadvantages. First, analog filters are not as accurately implemented as a digital filter due to variations in component tolerance. In addition, analog filters are subject to drift. More importantly, prior art analog filters are relatively fixed in the manner in which they compensate. In other words, a fixed phase and magnitude distortion is assumed in the system for recording and playing back the signal. An analog filter is designed which compensates for the assumed distortion. One problem with this approach is that phase and magnitude distortion produced by a system for recording and playing back ECG signals varies over time. It can vary as a result of wear on the heads for recording and playing back the signal. It can also vary in response to misalignment of the mechanical components which affect the record and playback head compliance with the tape. If the recorder is not matched with the playback unit, e.g., if they are made by different manufacturers, the filter may not compensate properly. Different recorders will therefore produce different results when the recorded data is played back on the same playback unit. Independently of time, distortion can vary as a result, e.g., of using an analog tape having a different thickness or type of tape coating thereon.
Prior art techniques for calibrating the ECG recorder include recording a series of rectangular calibration pulses of known amplitude on each of three channels used for recording ECG signals. Pulses are played back through a playback deck and passed through analog compensation filters to restore the pulse's original phase and frequency content. The playback unit is calibrated by measuring the amplitude of the calibration pulses and thereafter generating a data scaler which is applied to the ECG data.
There are several problems with this approach. One is that phase and magnitude distortion can vary from channel to channel. Also, generating a data scaler does not compensate phase and magnitude over frequency. Another problem results from a phenomenon called tape dropout. Tape dropout is an apparent reduced recording level on a tape caused by variations in the recording medium. When such areas are encountered during the calibration process, accuracy of calibration is adversely affected.